I am trying to learn how to use awesome as my WM. When I play a video in fullscreen, ex. in youtube, in my browser, it looks like a slide show. It sticks every 1 second.
But in xfce4, it plays smoothly.

Are you using compositing in Awesome? That'll bring video down to a crawl no matter how good your hardware is.

I don't think so. But I don't know if the xfce4 compositing settings will influence awesome.
in fact, I just installed awesome. use opera to watch youtube. when I use firefox to open a video in youtube and watch it in fullscreen, it works a little better but still sticks a bit. But in xfce4, There is no such problem at all.